Transbay BART service was extensively delayed Thursday evening by an equipment problem on a train between Oakland and San Francisco, officials said.

The unspecified issue was reported about 8:30 p.m. in the Transbay tube between West Oakland and Embarcadero stations, officials said. Westbound trains headed toward San Francisco International Airport, Daly City and Millbrae were said to be experiencing major delays.

While officials said service began to recover about 9 p.m., they did not know when riders could expect service to return to normal.